The present invention relates an optical recording medium, a method of recording information, a method of reproducing information, an apparatus for recording information, and an apparatus for reproducing information.
On the conventional optical recording medium, information is recorded in a two-dimensional fashion, that is, in a data-recording surface. It is proposed that information be recorded not only in the data-recording surface but also in the thickness direction of the medium at a density similar to the in-plane recording density. In other words, bits of information should be recorded in the thickness direction, at intervals as short as the wavelength of the light beam applied to record the information. If this is achieved, the recording capacity of the medium will increase remarkably.
Kawada, Microoptics News, Vol. 14, No. 1 (1996) discloses a method of recording information in the thickness direction of a medium. In this method, a lens is driven, moving the focal point in the depth direction of the medium, thereby recording information bits at positions in the medium which are aligned in the depth direction. The method can record information in the thickness direction, at a density of about 30 bits for 0.3 mm.
Each information bit is recorded by virtue of the absorption of light occurring at the point where the light beam is focused by moving the lens. The efficiency of light absorption and the distance the lens is moved limit the thickness of that part of the medium which serves the depth-direction recording. Further, the speed of recording information cannot be increased so much since the lens must be moved to record information bits in the depth direction of the medium.
No conventional method is available that is relatively simple and can yet record information bits in the thickness direction of a medium at a density substantially equal to the reciprocal of the wavelength of the light beam applied to the medium. Moreover, no conventional method can record information in the depth direction of a medium at a sufficiently high speed.